The present invention relates to a process of preparing a catalyst supporting highly dispersed metal particles utilized in the field of catalyst.
While support-type metal catalysts have been widely employed in various chemical reactions, it is required to reduce the particle diameter of the metal particles as much as possible for uniformly supporting the metal particles on a support in order to effectively utilize the catalytically active metals especially when such an expensive catalyst metal as a precious metal is employed. In order to attain this requirement, many attempts have been made to support a metal of fine particles on a support.
Heretofore, a metal ion in a solution has been reduced by means of a reductant to its metallic state to form metal particles. In general, in order to obtain the metal fine particles, the reduction is proposed to be carried out under the conditions that many nuclei are made to be formed of which subsequent growth is depressed. However, in practice, the metal particles once generated are likely to agglomerate with the increase of their particle size. It is quite difficult to produce metal particles of monodispersion having narrow particle size distribution of which particle size is not more than 30 .ANG.. Moreover, the amount of the metal particles capable of supporting on the support in the monodispersion state is restricted depending on the surface area of the support on which the metal particles are supported. It is difficult to support the metal particles on a support having a low specific surface area in the monodispersion state which invites more excellent catalytic effects.
Although, in order to overcome these problems, the metal particles are obtained by adding a polymer surfactant or such a substance as sulfur colloid as protective colloid to a solution containing a metal-containing ion to reduce the said metal containing ion, this process possesses the following disadvantages. (1) The lower limit of the metal particles is about 30 .ANG., (2) the degree of monodispersion on the support is poor, and (3) the metal particles produced are difficult to be separated from the remaining protective colloid, and during the washing to remove the colloid, the particles are likely to agglomerate with each other to widen the particle size distribution range.